Conventional edge-emitting laser diodes are well known. In these diodes, laser radiation is emitted in a plane that is a continuation of the plane of the p-n junction that forms the diode. Different types of these diodes are widely used to provide laser radiation in the infrared and visible regions. While these diodes have enjoyed considerable commercial success, they are relatively large and, as a result, are difficult to integrate with other devices.
Unlike the edge-emitting laser, the Vertical Cavity Surface Emitting Lasers (VCSELs) emit laser radiation in the direction perpendicular to the plane of the p-n junction formed in the laser diode. Considerable information concerning the structure and formation of such laser diodes is set forth, for example, in U.S. Pat. No. 4,949,350; in J. Jewell et at., "Microlasers," Scientific American , Vol 265, No. 5, pp. 86-94 (November 1991); in J. Jewell et al., "Vertical-Cavity Surface Emitting Lasers: Design, Growth Fabrication, Characterization," IEEE Journal of Quantum Electronics, Vol. 27, No. 6, pp. 1332-1346 (June 1991); in G. R. Olbright et al., "Cascadable Laser Logic Devices: Discrete Integration of Phototransistors with Surface-Emitting Laser Diodes," Electronics Letters, Vol. 27, No. 3, pp. 216-217 (Jan.31, 1991); and in J. Jewell et al., "Vertical Cavity Lasers for Optical Interconnects," SPIE Vol. 1389 International Conference on Advances in Interconnection and Packaging, pp. 401-407 (1990 ), all of which are incorporated herein by reference.
As set forth in certain of the above-referenced publications, vertical cavity lasers have numerous advantages over edge-emitting lasers, some of the most important of which are that they can be fabricated in extremely small sizes, e.g., on the order of one micrometer in diameter, and can be readily integrated with other devices such as transistors.
To date, however, application of vertical cavity lasers have been limited by the absence of any vertical cavity laser which emits visible laser radiation. This invention is the realization of visible VCSEL because of the difficulties associated with the growth of InAlGaP materials, as well as the VCSEL device structure. Indeed, integration of high efficiency InAlGaP quantum well or bulk layer active material with high reflectivity epitaxial mirror structures, with the requisite precision necessary for low-threshold lasing, represents a particularly complex and demanding materials growth challenge. Precise specification of the optical properties of the active region material is critical because of the very high gain required in VCSEL structures and the unique materials difficulties associated with the InAlGaP system. Extreme difficulty in obtaining low-resistivity p-type InAlGaP with high Al concentrations places constraints on the Al composition in the InAlGaP cladding layers in visible edge-emitting lasers and, consequently, lead to reduced heterojunction band offsets and less efficient carrier confinement.